Wait for your string beans to fully mature and begin to turn brown on the vine. Do this just for the beans you want to save. Harvest the ones you want to eat before they turn brown.
Pluck the string beans off of the vine. Open up the pods to reveal the beans inside.
Take the beans out of the pods and set them on a paper plate. Allow them to completely dry out. It should take one to three days, as they are slightly dry when you pick them.
Set your dry beans in an envelope and seal it shut. Write "string beans" on the front of the envelope if you are saving more than one type of seed.
Place the envelope in a cool, dry place. Plant them in the spring when the danger of frost has passed.